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Technology of the sea - offshore wind turbines

WORLDWIDE: Nearly all turbines that operate offshore or are currently being installed feature a conventional fast-speed geared drive system. One 1.5MW Goldwind direct drive turbine operating in Chinese waters is a known exception.

The GE 4.1-113 turbine, is GE's first offshore turbine since the early 2000s

By the end of 2010 fewer than 40 turbines in the 5MW class were installed offshore, all from German firms. They include 14 units supplied by Repower, six Multibrids from Areva Wind, and 15 from turbine manufacturer and offshore developer Bard, half of the latter being grid-connected. Almost all these are part of the major 80-turbine Bard Offshore I North Sea wind farm, under construction in German waters. Shortly to follow are 30 Repower 5MW turbines to be installed at Ormonde, in the Irish Sea off Cumbria, UK, this year. And for 2012/13 Repower has slated the uprated 6.15MW 6M model for two major offshore projects, each with 48 turbines.

Large rotor diameter

Offshore wind turbine models in development have power ratings that range between 4.1 and 10MW, and comprise a mix of direct drive, conventional fast-speed geared and medium-speed geared concepts. The bulk of these products are in the 6MW+ class, a size that looks likely to emerge as a temporary standard for offshore turbines. Within this segment and for bigger power ratings a prominent trend seems to be much larger rotor diameters compared to earlier turbine designs.

The largest rotor size operating offshore, 126 metres, is from Repower's 5M/6M series, but German manufacturer Nordex has announced a 6MW turbine with a 150-metre rotor diameter, and US firm Clipper's 10MW Britannia turbine features a similar sized rotor.

Explaining its product development strategy, Nordex describes a lighter, more powerful third-generation offshore turbine technology characterised by 6MW power rating and a top head mass (nacelle + rotor) of about 55 tonnes per MW. The weight of the turbine head is an improvement on the 70–90 tonne per MW quoted for the relatively heavy and expensive second-generation 5MW turbines. The company says that at sites with an average 10 metre per second (m/s) wind speed, its new N150/6000 will generate 17% more energy a year compared to a standard 6MW turbine with a 125-metre rotor.

Fitting into the same trend is the V164-7.0MW turbine in development from Denmark's Vestas. It has a 164-metre rotor diameter, resulting in a power output per square metre comparable with the Nordex N150. The 10MW SeaTitan, from US-based AMSC Windtech — designed with a high-temperature superconductor generator and planned for 2013 — now features a 190-metre rotor diameter, where previously it was 164 metres. This trend, as well as its higher yield potential, is primarily aimed at substantially pushing down the cost of energy, a calculation that includes capital and operational costs as well as energy production over a turbine's lifetime.

Drive system preferences

Direct-drive systems hold the promise of greater relaibility through fewer components. At least seven suppliers have developed or are developing conventional direct-drive turbines between 4.1 and 10MW. US manufacturer GE's model is 4.1MW and China's XEMC Darwind's is 5MW. French maker Alstom, China's Goldwind, and Germany's Siemens and Nordex are all producing 6MW models. Norwegian firm Sway Turbine has the largest at 10MW. GE and Nordex have further opted to position the ring generator behind the tower, which can prove easier than a common front mounting when servicing. Sway has chosen an unusually large generator diameter, rumored to be 20 metres or more.

Earlier this year, Bard installed two 6.5MW prototypes with a new so-called distributed gearbox concept with two generators. An alternative 6.5MW drive solution based on a hydraulic power converter enables a combination of variable rotor speed and a fixed-speed generator that connects directly to the grid, thus eliminating the need for an electronic power converter.

Hyundai of Korea and Dongfang of China are planning 5.5MW offshore prototypes featuring geared drive systems this year, while Sinovel of China has a 6MW geared turbine in development. All three projects are co-developments with AMSC Windtec.

Medium-speed gearboxes

Clipper, for its 10MW Britannia development project, had initially planned a distributed drive system with four generators, as with its smaller 2.5MW Liberty turbine model. However, according to wind industry sources, the company has changed to a medium-speed gearbox and single generator.

The G11X-5MW machine from Spain's Gamesa is technically based on the G128-4.5MW onshore model, with a two-stage medium-speed gearbox and permanent magnet generator (PMG). The model in development by Vestas will apply a three-stage medium-speed geared in a combination with a PMG rated at 400rpm. The only commercially available medium-speed offshore turbine is Areva Wind's 5MW Multibrid M5000, with a single-stage gearbox and a PMG rated at 150rpm. Its low speed and compact drive system could enhance reliability.

Two-blade renaissance

While all the horizontal-axis offshore turbine models described here comprise an upwind rotor with three blades, for offshore application there could be a renaissance of two-blade machines. with two-blade turbines a fully assembled nacelle with rotor can be transported on a ship deck, and put on top of a ready tower in a single hoisting action that saves time and costs. And the rotor can be fixed horizontally for easier and potentially safer access by helicopter.

The disadvantages, however, of rigid two-blade rotors are that they are dynamically unbalanced, and have a higher aerodynamic noise level as they typically turn faster than three-blade equivalents. But noise is less of a restriction offshore, while dynamic imbalance issues can be countered with a flexible blade mounting, called a teeter hub, and perhaps by other means.

Dutch firm 2-B Energy is developing a 6MW fast-speed geared downwind two-blade turbine, in which the rotor turns behind the tower, UK-based Condor Wind Energy is working on a 5MW two-blade upwind-type geared turbine with a limited pivoting or "teeter" hub. Finally, German engineering firm Aerodyn has developed a 6.5MW two-blade downwind model for SCD Technology, also German-based, with a 140-metre rotor diameter and a top head mass of only 205 tonnes. Its Chinese partner Mingyang is planning to install a prototype in China.

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